In the traditional public switched telephone network (PSTN) hierarchy, a tandem toll center. Class 4 offices serve to interconnect Class 5 offices, or central offices (COs) not interconnected directly.As the lowest class of toll center, Class 4 offices interconnect within a relatively local toll network and provide access to higher-order toll centers. In the contemporary PSTN, a Class 4 office commonly serves as a Class 5 office, as well, with the separate functions provided through logical and physical partitioning within the switch. See also Class 4/5, Class 5, CO, and PSTN.